StarDate Podcast

The closer we look at the worlds of the solar system, the more places we see that could be homes for life. Some of those worlds orbit Jupiter, the largest planet in the solar system. Jupiter itself isn’t on the list. It’s a big ball of gas with no solid surface. There has been speculation that large organisms could float through its skies. But that’s considered a long shot. It’s more likely that life could inhabit some of Jupiter’s moons. The leading candidate is Europa. It’s about the same size as our own moon. A deep ocean of liquid water probably lies below its icy crust. Plumes of hot water may squirt into the bottom of the ocean. The plumes would contain a variety of compounds – perhaps including the chemistry of life. So Europa has the right combination of water, heat, and chemistry to support life – at least microscopic life. Europa isn’t the only Jovian moon with a deep ocean. The largest moon, Ganymede, may have more liquid water than all Earth’s oceans combined. One other big moon may have an ocean as well. But the crusts of these moons are much thicker than Europa’s. So even if their oceans are inhabited, it’ll be much harder for us to find evidence of life. Look for Jupiter in the eastern sky in early evening, and arcing high across the sky later on. It looks like a brilliant star. Through binoculars, its big moons look like tiny stars quite close to the planet. More about Jupiter tomorrow. Script by Damond Benningfield

Jupiter looks like it’s wearing zebra stripes. Bands of clouds that run parallel to the equator alternate between bright and dark – zebra stripes. Each one is thousands of miles wide. The stripes are a result of Jupiter’s composition and its rotation. It’s basically a ball of gas – it’s made almost entirely of hydrogen and helium. And even though it’s 11 times the diameter of Earth, it spins on its axis in less than 10 hours. That forces the clouds that top its atmosphere into bands that stretch from east to west. The bands alternate between belts and zones. The belts are darker – probably because they allow us to see deeper into the atmosphere. The zones are topped by the highest clouds. The clouds are made of frozen ammonia, which looks bright white. The belts don’t have that layer. Instead, we’re seeing clouds in the next layer down. Those clouds are made of water and other compounds, which are darker. The stripes are flanked by jet streams that blow in alternating directions. They can roar at hundreds of miles per hour. They keep the belts and zones separated – maintaining the zebra stripes on this giant planet. Jupiter is at its best this week. It’s in view all night, and it shines brightest for the year. It looks like a brilliant star. It’s low in the eastern sky in early evening, and climbs high across the sky later on. The stripes are easily visible through just about any telescope. Script by Damond Benningfield

If today is your birthday, then Happy Birthday! The next one is just one year away – 365 sunrises and sunsets. If today is your birthday and you happen to be from Jupiter – well, Happy Birthday, and … we’re sorry. Your next one is almost 12 Earth years away – almost 10,500 sunrises and sunsets. The Jovian year is so long for a couple of reasons. First, the planet is more than five times farther from the Sun than Earth is. So its path around the Sun is more than five times longer than Earth’s. The second reason is the laws of orbital motion. The farther a planet is from the Sun, the slower its orbital speed. At Jupiter’s great range, it moves at less than half the speed of Earth. Ergo, one Jovian year lasts almost 12 Earth years. But to get all those sunrises and sunsets, you also have to factor in the length of a Jovian day. Although Jupiter is 11 times the diameter of Earth, it spins in a hurry – a day lasts less than 10 hours. Add it all up, multiply, divide, and carry the two, and – well, it’s a lot of days between birthdays on the Sun’s largest planet. Jupiter is especially vibrant now. It reaches opposition this weekend – it lines up opposite the Sun in our sky. It rises around sunset and is in view all night. The planet is also closest to us, so it shines at its brightest. In fact, in all the night sky right now, only the Moon outshines it. More about Jupiter tomorrow. Script by Damond Benningfield

Stars are born when giant clouds of gas and dust break apart and collapse. And if that’s all there was to it, the Milky Way Galaxy would give birth to a couple of hundred stars every year. Instead, thanks to feedback from the stars themselves, it makes only a few. Feedback is a process that clears away the material for making stars, but can also trigger the birth of more stars. Young stars, for example, produce winds and jets that blow away the gas and dust around them. Since stars are born in clusters, many youngsters can be sweeping away the star-making material at the same time. That pares back the number of stars that can be born in a cluster. Mature stars add to the feedback – not only with winds, but also with radiation. Hot stars generate a lot of ultraviolet energy. It vaporizes tiny particles of dust – eliminating possible building blocks for new stars. The heaviest stars explode as supernovas. These blasts can clear out the space for light-years around, creating big, empty bubbles. And supernovas also accelerate subatomic particles around them to almost the speed of light. These “cosmic rays” help to sweep away the raw material for making more stars. But supernovas can also enhance the birth rate. Their shock waves can cause distant clouds of gas and dust to collapse to form stars. So feedback is a complex process – one that both aids and hinders the birth of new stars. Script by Damond Benningfield

The planet Venus is switching sides today – sides of the Sun. It’s crossing behind the Sun as seen from Earth, so it’s moving from the morning sky to the evening sky. But we won’t be able to see it for several weeks. Venus is the second planet from the Sun, while Earth is third. So Venus crosses both behind the Sun and between Earth and the Sun. It switches between Morning Star and Evening Star appearances each time. Each of these crossings happens every 584 days – about 19 and a half months. The planet spends about eight months in both the morning and evening sky, and disappears from view during the crossings. When Venus passes between Earth and the Sun, it’s closest to us, so it moves across the sky quickly – it’s hidden in the Sun’s glare for only a few days. When it’s behind the Sun, it’s farthest – about 160 million miles. Because of the relative motions of Earth and Venus, it moves across the sky quite slowly. So it remains hidden in the light for three months or so. Depending on your location, Venus could emerge as the Evening Star as early as mid- to late February. It’ll be quite low in the twilight, so it won’t be easy to find. The planet will climb into better view in early March. Venus will reign over the evening sky until October, when it will vanish in the sunlight as it once again switches sides. Tomorrow: slowing down the stellar birth rate. Script by Damond Benningfield

The gibbous Moon soars across the sky tonight. It’s about three days past full, so the Sun lights up about 90 percent of the lunar hemisphere that faces our way. That makes the Moon nice and bright. But it’s not as bright as you might expect. In fact, it’s only about half as bright as the full Moon. There are a couple of reasons for that. One is our viewing angle. The full Moon stands opposite the Sun in our sky, so the sunlight that strikes it is reflected straight back toward Earth. That makes the Moon a more efficient mirror. But the main reason is the shadows. At full Moon, the shadows on most of the visible surface are short. In fact, there are almost no shadows at all across the center of the lunar disk. But as the Moon moves in its orbit around Earth, the angle between the Sun and Moon changes. The Sun drops lower in the lunar sky, so the shadows grow longer as seen from Earth. More shadows mean a darker surface. Despite appearances, none of the Moon is especially bright. It reflects only a bit more than one-tenth of the sunlight. It looks so bright only because it’s a close, big presence – lighting up the night sky. A bright star joins the Moon tonight: Regulus, the heart of the lion. It’s below the Moon as they climb into good view, about 9 or 9:30. The Moon will slide toward the star during the night, and they’ll be especially close as the dawn twilight begins to erase the star from view. Script by Damond Benningfield

There’s no fountain of youth to make people look younger. But there is one for stars. It’s a process that sounds like something from a horror movie – “stealing” life from another star. A good example is in Fornax, the furnace, which is low in the south at nightfall. The constellation has only one moderately bright star, Alpha Fornacis. It’s 46 light-years away. To the eye alone, it’s not much to look at. But binoculars reveal two stars. One of them is bigger and heavier than the Sun. Because of its greater mass, it’s nearing the end of its life, even though it’s almost two billion years younger than the Sun. The other visible star is smaller than the Sun, and its surface is cooler, so it glows orange. Yet it should be even redder than it is. And that’s where the story of rejuvenation comes in. The star is a blue straggler. That means its color has shifted to bluer wavelengths. That might be because it merged with another star. The merger would rev up the nuclear reactions in its core, making it hotter and bluer. On the other hand, it might have changed color by simply stealing gas from a third star in the system. This extra star was discovered in 2016. It’s a white dwarf – a stellar corpse. It’s about half as massive as the Sun, and it’s quite close to the blue straggler. So the straggler might have siphoned away the star’s life – taking some of its gas to “rejuvenate” its own appearance. Script by Damond Benningfield

The Moon sometimes rumbles during “moonquakes.” And according to a recent study in China, those quakes may happen fairly often. The first moonquakes were recorded by instruments left on the lunar surface by Apollo astronauts. Some of the quakes are deep – they’re centered hundreds of miles below the surface. They’re triggered by the tides – the gravitational pull of Earth squeezes and stretches the interior, causing things to clatter about. The other main moonquakes are shallow – they occur much closer to the surface. These quakes are triggered by the Moon itself. Our satellite world is shrinking as it loses its internal heat. It might have shrunk by as much as 150 feet over the past few hundred million years, and continues to contract even today. The Chinese study looked at 74 spots on the lunar surface, on both the nearside and farside. Scientists pored over hundreds of pictures snapped from 2009 to 2024. And they found 41 fresh landslides that happened during that period. They ruled out other causes for about 70 percent of the landslides. That left them with one conclusion: the landslides were caused by shallow moonquakes. So the Moon continues to shake and jiggle long after its birth. The Moon has some prominent companions tonight. It’s flanked by the brilliant planet Jupiter and the star Pollux, the brighter “twin” of Gemini. Castor, the other twin, is to the upper left of the Moon. Script by Damond Benningfield

Nothing symbolizes a cold, moonlit night like the howl of a wolf. The haunting sound can travel for miles. And if you live around wolf territory, you might especially notice it tonight. There’s a full Moon – the Frost Moon, Moon After Yule, or Wolf Moon. Despite what you might think, though, the wolves aren’t actually howling at the Moon. Many cultures have associated wolves and the Moon – ancient Moon goddesses often were depicted hanging with wolves. And biologists say that wolves may howl more around the time of the full Moon. But that’s only because they’re creatures of the night, so they’re more active when there’s more moonlight. Wolves communicate with each other in many ways besides howling. They growl, bark, and whimper. Each method conveys a different type of message. And howls can have different meanings, too – conveyed through changes in pitch, duration, and frequency. The howls help them attract mates, coordinate their hunting, and warn members of other packs to stay away. There’s even a “lonesome” howl when a wolf gets lost. Wolves do tilt their heads up when they howl – as though they were talking to the Moon. But there’s a practical reason – the sound carries farther. So if you happen to hear the lonesome howl of a wolf under the light of the full Moon, enjoy the serenade – just don’t think the wolf is howling at the Moon. Script by Damond Benningfield

At the dawn of the 19th century, the celestial police were on patrol. They were looking for a planet between the orbits of Mars and Jupiter. And on the century’s first day, a future squad member found one – sort of. Later discoveries showed that it wasn’t a planet at all, but the first and largest member of the asteroid belt – a wide band of millions of rocky bodies. Astronomers were looking for a planet because of the numbers. There seemed to be a mathematical relationship between the distances from the Sun to the known planets. But there was a gap between Mars and Jupiter. So one astronomer began organizing a search party: the celestial police. Giuseppe Piazzi, at the Palermo Observatory in Sicily, was on the list of people to invite. But he was already searching on his own. And before he got his invitation, he found something – 225 years ago today. Piazzi originally thought it was a comet – but hoped for something bigger. As other astronomers began studying it, they decided it was the sought-after planet. They named it Ceres, for the Roman goddess of agriculture. Within a few years, though, they’d found several other bodies in similar orbits. So they realized that Ceres wasn’t a planet at all, but just one member of a band of debris – the asteroid belt. Today, Ceres has regained its planetary status – sort of. It’s a dwarf planet – the only one in the inner solar system. Script by Damond Benningfield

By the time the ball drops in Times Square tonight, the people of the Line Islands will be almost a full day into 2026. The islands are in the Pacific Ocean, south of Hawaii. But they’re just across the International Date Line. That makes the islands the first place to see the new year. The Date Line is needed because the time gets an hour earlier for every time zone west, and an hour later for every time zone east. Without a place to reset the date, time just wouldn’t make sense. The line mostly runs down the middle of the Pacific – half way around the globe from Greenwich, England, which is the starting point for the time system. But individual countries can set their own time zones. So the line zigzags between Alaska and Russia. And near the equator, it jumps more than a thousand miles to the east. That extension came three decades ago. The island nation of Kiribati changed its time zones. That made it easier for the country to do business with Australia, which is west of the Date Line. The country’s easternmost extension is the Line Islands. So the date changes there first – making the Line Islands the first places on Earth to ring in the new year. American Samoa is farther west than the Line Islands. But its time zone puts it on the opposite side of the Date Line – making it one of the last places to change the calendar. Script by Damond Benningfield

The Sun and similar stars are losing weight – they blow some of their gas into space through strong “winds.” And at the end, they blow away all of their outer layers of gas. That leaves only their hot, dense cores, known as white dwarfs – tiny remnants of their once brilliant selves. An example is Sirius B, the faint companion of Sirius A, the brightest star in the night sky. Sirius climbs into view in the east-southeast by around 8:30 or 9, and arcs across the south during the night. Sirius B is too small and faint to see without a telescope. But long ago, that wouldn’t have been the case. The star probably was a few times as massive as the Sun, so it would have shined brighter than Sirius A is today. Such a hot, bright star produces a much thicker wind than the Sun does, so it loses mass at a higher rate. And because Sirius B was heavier than the Sun, it burned through the nuclear fuel in its core much faster – it fizzled out in a couple of hundred million years, while the Sun is still only half way through its 10-billion-year lifetime. As it neared the end of its life, Sirius B puffed up like a giant balloon, then ejected its outer layers. Some of that gas probably piled on the surface of Sirius A, increasing its mass. Today, Sirius B is as heavy as the Sun, but only as big as Earth. It still shines because it’s extremely hot. But it’s only a faint reminder of its former glory. Tomorrow: an early new year. Script by Damond Benningfield

Over the centuries, we’ve given all the visible stars many names – proper names, catalog designations, and others. But only one star is best known not by any of its formal names, but by its nickname: the Dog Star. Its proper name is Sirius, and it’s the leading light of the constellation Canis Major, the big dog – hence the nickname. Sirius is so well known because it’s the brightest star in the night sky – its closest competition is only about half as bright. Part of that is because Sirius itself is a couple of dozen times brighter than the Sun. But part of it is because Sirius is one of our closest neighbors – less than nine light-years away. And thanks to the relative motions of Sirius and the Sun, Sirius is moving closer, at about 12,000 miles per hour. It’ll continue to close in for tens of thousands of years. But the distances between stars are so vast that even at that speed, Sirius won’t grow much brighter in our sky. Astronomers discovered the star’s motion toward us by measuring its Doppler shift – a slight change in the wavelength of its light. The Doppler shift also allowed them to measure the orbit of a faint companion – a stellar corpse known as a white dwarf; we’ll have more about that tomorrow. In the meantime, look for Sirius climbing into good view in the east-southeast by around 8:30 or 9. It’s directly below the three stars of Orion’s Belt, so you can’t miss it. Script by Damond Benningfield

The most important thing to know about a star is its mass – how heavy it is. Among other things, the mass reveals how long the star will live and how it will die. Measuring the mass of a single star is tough. It’s a lot easier to get the masses of stars in binary systems – two stars that orbit each other. An example is Menkalinan, the second-brightest star of Auriga. It’s a third of the way up the northeastern sky at nightfall, below the charioteer’s brightest star, Capella. Menkalinan’s two stars are so close together that we can’t see them as individual points. But breaking the system’s light apart reveals the presence of both stars. The stars orbit each other every four days, at about one-tenth of the distance from Earth to the Sun. Combined, those numbers reveal the system’s total mass. A couple of other numbers complete the picture. One is the angle at which we’re seeing the system. In the case of Menkalinan, that’s easy – the stars pass in front of each other, so we see the system edge-on. The other is the orbital motions of the stars. Plugging those numbers into the formula provides a precise mass for the individual stars. The stars of Menkalinan are almost identical. Each is more than twice the mass of the Sun. Each is also bigger and brighter than the Sun. So even though Menkalinan is more than 80 light-years away, it’s easy to see – the combined glow of two big, well-understood stars. Script by Damond Benningfield

Orion is climbing into prominence in winter’s evening sky. The hunter clears the eastern horizon by about an hour and a half after sunset. He’s led by his shield. It’s not as easy to see as his belt or other features. But the shield’s brightest star does stand out. Pi-3 Orion is in the middle of the shield – where Orion’s hand is holding it. The star is a little bigger, heavier, and hotter than the Sun. That makes it about three times brighter than the Sun. There are a couple of ways to look at that brightness: apparent magnitude and absolute magnitude. Apparent magnitude is how bright a star looks. In that scale, Pi-3 shines at about magnitude 3.2 – not especially bright, but bright enough to see under even most light-polluted skies. But that number doesn’t tell you the star’s true brightness. It might be especially bright, but it might also be especially close. So that’s where absolute magnitude comes in. It’s how bright a star would look at a distance of 10 parsecs – 32.6 light-years. If you lined up every star at that distance, you could easily tell which ones are truly bright. Pi-3 is just 26 light-years away. If you moved it out to 10 parsecs, it would shine at magnitude 3.65 – half as bright as it looks now. In fact, if you moved all the stars in the shield to that distance, Pi-3 would be its faintest member – a middling middle for the shield. Script by Damond Benningfield

Not many planetary spacecraft get to shower off. But the Cassini spacecraft did – more than once. It flew through plumes of ice and water vapor from Enceladus, a moon of Saturn. The encounters helped scientists confirm that an ocean hides below the moon’s icy crust. Enceladus is a little more than 300 miles in diameter – roughly the distance from Los Angeles to San Diego. Its surface is completely coated with ice. That makes it the most reflective large body in the solar system, so it looks bright white. Much of that ice comes from more than a hundred geysers near the moon’s south pole. They erupt from deep cracks in the crust. They contain water vapor, water ice, hydrogen, grains of salt, and other compounds. Much of this material falls back on the surface. The rest of it escapes into space, where it forms a thin ring around Saturn. The geysers erupt from a global ocean. It’s buried about 20 to 25 miles below the surface, and it could be 10 miles deep or more. Hot, mineral-rich water could flow into the ocean through fissures on its floor. So the ocean appears to offer all the ingredients for life: liquid water, minerals, and a source of heat. That makes Enceladus a high-priority target in the hunt for life beyond Earth. Saturn is near our own moon this evening. It looks like a bright star, shining steadily through the lunar glare. But you need a good-sized telescope to pick out Enceladus. Script by Damond Benningfield

A look at the evening sky is a nice way to wrap up your Christmas. It features the Moon, two bright planets, and some of the brighter stars in all the night sky. As twilight drains from the sky, the Moon is well up in the southwest. The Sun lights up more than a quarter of the lunar hemisphere that faces our way, so it’s a fat crescent. It’s waxing toward first quarter, on Saturday. The planet Saturn is to the upper left of the Moon, and looks like a bright star. It shines so brightly for a couple of reasons: It’s the second-largest planet in the solar system – more than nine times the diameter of Earth – and it’s topped by clouds that reflect much of the sunlight that strikes them. The only planet that’s bigger than Saturn is Jupiter, and it climbs into good view, in the east-northeast, by 7 or 7:30. In all the night sky right now, only the Moon outshines it. The “twin” stars of Gemini – Pollux and Castor – stand to Jupiter’s left and upper left. At the same time, the brilliant constellation Orion is off to the upper right of Jupiter. Look for its three-star belt aiming straight up from the horizon, flanked by orange Betelgeuse and blue-white Rigel. Taurus perches well above Orion. It’s marked by its bright orange eye, Aldebaran. And the Dog Star, Sirius, climbs into good view by 8 or 8:30, below Orion’s Belt. It’s the brightest true star in the night sky – a beautiful decoration for Christmas night. Script by Damond Benningfield

For a star, showiness comes with a price. The most massive stars are far brighter than their punier cousins. But they live much shorter lives. An example is Alpha Camelopardalis. It’s the third-brightest star of Camelopardalis, the giraffe. It’s dimmed by its great distance – about 5500 light-years – so you need a dark sky to see it. Even so, it’s one of the most remote stars visible to the eye alone. The star is impressive. It’s more than 30 times the diameter of the Sun, and almost 40 times the Sun’s mass. Because of that great heft, Alpha Cam “burns” through the nuclear fuel in its core in a big hurry. That makes its surface tens of thousands of degrees hotter than the Sun’s, so the star shines blue-white. The combination of size and temperature makes Alpha Cam more than 600,000 times brighter than the Sun. The price for that showiness is a short lifespan. Stars like the Sun live for billions of years. But Alpha Cam will stick around for only a few million years. So even though it’s only about two million years old, its days are numbered. Before long – astronomically speaking – it will expire. Just how it will go out isn’t clear. Its core may collapse to form a black hole, with its outer layers exploding as a brilliant supernova. On the other hand, the entire star may collapse, forming a heavier black hole – a dark ending for a dazzling star. Script by Damond Benningfield

If you’d like to know how dark your night sky is, then look high in the northeast after the Moon sets this evening for the stars of Camelopardalis, the giraffe. If you can see any of them, then congratulations – your sky is pretty dark. Light pollution wipes out the view for most Americans. The glare of street lights, billboards, and other artificial sources overpowers the stars. None of the stars of Camelopardalis, for example, is brighter than fourth magnitude, which is pretty faint. So unless you’re under dark skies, there’s not much to see. That’s a little misleading, though. The giraffe’s brightest stars are all stunners. They look so faint only because they’re so far away. The giraffe’s brightest star is Beta Camelopardalis – Beta Cam for short. It’s a huge, massive star that shines roughly 1600 times brighter than the Sun. But it’s about 840 light-years away, so it’s a faint dot in the night sky. The next-brightest star is CS Cam. It is a supergiant star that’s perhaps 75,000 times the Sun’s brightness. But it’s 3400 light-years away. And the third-brightest, Alpha Cam, is the most impressive of all: more than 600,000 times the Sun’s brightness. At a distance of 5500 light-years, it’s one of the most remote stars visible to the unaided eye – but only under especially dark skies. More about Alpha Cam tomorrow. Script by Damond Benningfield

Space agencies are talking a lot these days about sending people to the Moon – and even setting up permanent bases there. But you might not want to be on the Moon seven years from today. A space rock that’s half the size of an NBA arena has a slight chance of slamming into the Moon. Asteroid 2024 YR4 was discovered last year, two days after Christmas, when the asteroid had flown just half a million miles from Earth. Early observations gave it more than a three percent chance of hitting Earth on December 22nd, 2032. As astronomers tracked it longer, they ruled out that chance. Instead, they calculated that it’ll pass about 7,000 miles from the Moon. But there’s a 45-thousand-mile margin of error. So there’s a better than four percent chance that it will hit the Moon. 2024 YR4 is so far away right now that we can’t see it. It won’t return to view until 2028. Once it reappears, astronomers will refine their calculations. That probably will rule out the chance of an impact. But for now, we can’t know for sure. The asteroid is about 200 feet in diameter. That’s about the size of the asteroid that gouged the famous meteor crater in Arizona. So an impact on the Moon probably would form a big crater. Debris from the impact could travel hundreds of miles – cosmic missiles crashing across much of the Moon. Script by Damond Benningfield

Today is the December solstice – the start of winter in the northern hemisphere. It’s the darkest time of the year – many hours of darkness for watching the stars. But it’s also a great time for space science in Antarctica, where it’s daylight around the clock. NASA launches high-altitude balloons from a base near McMurdo Station, the continent’s largest settlement. Their payloads can keep an eye on the heavens for weeks as they circle around the south pole. When their work is done, they parachute to the ice. Scientists from the United States, Japan, and other countries hunt for meteorites in Antarctica. There aren’t more meteorites there, but on the ice, there’s a good chance that almost any rock came from beyond Earth. Over the decades, tens of thousands of meteorites have been found there. Astronomers take advantage of the daylight to repair and upgrade telescopes at the south pole. The collection includes instruments that study the “afterglow” of the Big Bang. The instruments can operate even in daylight, but the southern summer is the only time to do most of the maintenance work. The south pole also is home to IceCube – a collection of thousands of light detectors frozen in the ice. They look for neutrinos – particles that tell us about some of the most energetic events in the universe. IceCube can also operate all year – even under the midnight sun at the south pole. Script by Damond Benningfield

If you don’t like winter but you live in the northern hemisphere, then give a little thanks to the laws of orbital mechanics. Because of Earth’s lopsided path around the Sun, winter is the shortest season north of the equator. Earth’s orbit around the Sun isn’t a perfect circle. Instead, it’s an ellipse. It looks like a flattened circle, with the Sun slightly away from the center. Because of that shape, our distance to the Sun changes. And that’s where the laws of orbital motion come into play. Johannes Kepler devised those laws more than four centuries ago. One of them says that if you draw a line from the center of the Sun to the center of a planet, as the planet orbits the Sun that line will sweep out equal areas over equal periods of time. To do that, a planet must move fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. Earth is closest to the Sun in early January – the start of winter – and farthest at the start of summer, in July. So Earth moves around the Sun in a hurry during northern winter, making the season shorter. This winter, for example, starts at 9:03 a.m. Central Time tomorrow. That’s the moment of the December solstice, when the Sun stands farthest south for the year. The season ends 89 days later. By comparison, this past summer lasted almost 93 days – a longer season thanks to the science of orbits. More about the solstice tomorrow. Script by Damond Benningfield

The “she-goat” is a lot more than it seems. What looks like a single brilliant star is actually two sparklers. Both of them are much bigger, heavier, and brighter than the Sun. Capella is the brightest star of Auriga, the charioteer. Its name comes from a Latin phrase that means the she-goat. It’s 43 light-years away – just down the block by astronomical standards. Both stars of Capella are about two and a half times as massive as the Sun. And both are more than 70 times brighter than the Sun. But the stars are quite close together – less than the distance from Earth to the Sun. So we can’t see them as individual points of light, even with the biggest telescopes. Astronomers discovered Capella’s dual nature with a technique called spectroscopy. It breaks a star’s light into its individual wavelengths. The spectrum of Capella shows two patterns of light. The patterns move back and forth as the stars orbit each other. The patterns reveal details about both stars – their surface temperature, composition, and more. From that and other details, we know that both stars have moved beyond the prime phase of life. Now, they’re in the giant phase. Both stars have puffed up to about 10 times the diameter of the Sun – two big, brilliant stars for the she-goat. Capella is a third of the way up the northeastern sky at nightfall. It’s one of the brightest stars in the entire night sky, so you can’t miss it. Script by Damond Benningfield

The tales that describe many of the ancient constellations can be romantic, tragic, heroic, or majestic. Some, on the other hand, are just weird. An example is Auriga. The constellation is low in the eastern sky at nightfall, and climbs high across the sky later on. It’s marked by a pentagon of stars. It’s easy to pick out thanks to the brightest member of that figure, Capella – one of the brighter stars in the entire night sky. Although Auriga is described as a charioteer, the character usually isn’t depicted with a chariot. But he is shown with a goat and her two kids on his shoulder. There are several versions of his story. In one, he was an early king of Athens. He was raised by the goddess Athena. Among other things, she taught him how to tame horses. He was so good at it that he became the first person to harness four horses to a chariot, like the chariot that carried the Sun across the sky. Zeus, the king of the gods, was so impressed that he placed the charioteer in the stars. The goat is represented by Capella. It isn’t a part of any of the legends of Auriga from Greek or Roman mythology. It may represent the goat that suckled the infant Zeus, who placed her and her children in the sky in gratitude. The goat and kids may once have formed their own small constellation. Today, though, they ride on the shoulder of the charioteer – who rides on nothing at all. More about the charioteer tomorrow. Script by Damond Benningfield

At the cusp of the 20th century, it seemed like contact with another world was just a matter of time. In fact, the French Academy of Sciences announced a prize for such a feat 125 years ago today. The winner would receive 100,000 francs. There was only one catch: Mars didn’t count. The prize was established by Clara Guzman in honor of her son. He was a follower of astronomer Camille Flammarion, who wrote extensively – and fancifully – about the Red Planet. Guzman excluded Mars from the competition because it seemed just too easy. Percival Lowell had popularized the idea that Mars was crisscrossed by canals – built by Martians to bring water from the poles to the planet’s deserts. Inventor Nicola Tesla had reported hearing possible radio signals from Mars. And many others thought that vast dark areas on Mars were covered with vegetation. Many schemes were proposed to contact the Martians. One suggested creating giant geometric shapes in Siberia. Another suggested digging the shapes into the Sahara Desert, filling them with kerosene, and setting them on fire. None of the schemes ever materialized. And no one ever claimed the prize for contacting another world. So the French academy decided to award the prize for making physical contact. In 1969, it awarded the Guzman Prize to Apollo 11 astronauts Neil Armstrong, Buzz Aldrin, and Michael Collins – the first men to set foot on another world. Script by Damond Benningfield

The planet Mercury is shrinking. It’s contracted by several miles since its birth. And it’s continuing to get smaller even now. Mercury is the closest planet to the Sun. It’s also the smallest major planet in the solar system – a little more than 3,000 miles in diameter – about the width of the 48 states. It has a core of iron and nickel, surrounded by dense layers of rock. And it’s topped by a thin crust. The surface of the planet is marked by lots of impressive cliffs. The biggest is more than 600 miles long and about two miles high. They formed as Mercury lost heat from its interior. As the planet cooled, it shrank. Estimates of how much it’s contracted have ranged from about a mile to about nine miles. A recent study narrowed the range a little bit. It measured the most dramatic features, then scaled that to the surface of the entire planet. The result suggests that Mercury has shrunk by about three to five miles as a result of its cooling. And when you add in some other causes, the total contraction is about four to seven miles. And Mercury is still getting smaller today. This incredible shrinking planet is quite low in the southeast in the dawn twilight for the next few days. It looks like a bright star, but you need a clear horizon to spot it. And because of the viewing angle, it’s easier to spot from more southern latitudes. Tomorrow, the Moon stands to its right or upper right. Script by Damond Benningfield

Here’s what we know for sure about the planet K2-18b. It’s about 125 light-years away. It’s bigger and heavier than Earth. It orbits a cool, faint star once every 33 days. It receives about the same amount of energy from its star as Earth gets from the Sun. And it has an atmosphere. After that, things get muddled. Astronomers aren’t sure about the structure of the planet or the make-up of its atmosphere. And ideas about whether it might be habitable are all over the place. The confusion highlights the challenges of studying planets in other star systems. K2-18b passes in front of its star on every orbit. And as it does so, the chemical “fingerprints” of its atmosphere are added to the starlight. Substracting the starlight provides a profile of the atmosphere. But the profile is hard to read. Many of the fingerprints are subtle, and can be produced by different compounds. Earlier this year, a team announced the discovery of compounds in the atmosphere that could be produced by microscopic life. Follow-up studies by other groups contradicted that finding. But the original study team has stuck by its conclusions. So it’ll take a lot more work to know for sure what’s going on at K2-18b. The K2-18 system is in Leo, which climbs into good view after midnight. K2-18 is to the right of Denebola, the star that marks the lion’s tail. But it’s too faint to see without a telescope. Script by Damond Benningfield

For stars that are similar to the Sun, the end comes in stages. And each stage is triggered by changes in the star’s core. One star that’s going through those changes is Diphda, the brightest star of Cetus, the sea monster. The star is several hundred million years old – billions of years younger than the Sun. For most of its life, it “fused” hydrogen atoms in its core to make helium. When the hydrogen was gone, it began fusing hydrogen in a shell around the core. That made the star puff up, so it was classified as a red giant. Now, it’s finished off the shell, so it’s fusing the helium in the core to make carbon and oxygen. This phase is generally lumped into the red-giant category. Technically, though, it has its own name: the red clump. In a hundred million years or so, Diphda will have used up all the helium. The star isn’t massive enough to fuse the carbon and oxygen to make heavier elements. Without that energy, the core will collapse to about the size of Earth. It’ll be extremely hot, though, so it’ll blow away Diphda’s outer layers. For a while, the star will enter one more phase: a planetary nebula – a colorful cloud of gas and dust. When the cloud disperses, only the dead core will remain: a white dwarf – the hot but tiny remnant of a star. Cetus spreads across the southeastern quadrant of the sky at nightfall. Diphda is near its lower right corner, roughly a third of the way up the sky. Script by Damond Benningfield

People collect all kinds of things, from baseball cards to Persian rugs. Over the past 40 years, some NASA aircraft have collected dust – grains of dust from beyond Earth. Many of the collection efforts have taken place during meteor showers. That’s included the Geminid shower, which is at its peak tonight. A meteor shower takes place when Earth flies through a trail of particles that were shed by a comet or asteroid. Many of the particles burn up in the upper atmosphere, creating the streaks of light known as meteors. But many more grains are too small to burn up. They float down through the atmosphere. Some of them stop at a height of about 10 miles. And that’s where the research aircraft head. Once there, they open up small boxes that catch whatever is drifting along – pollen grains, parts of bugs, bits of volcanic ash, and even exhaust from rocket engines. Analysis reveals whether the captured particles are from Earth or from outside. The cosmic particles can then be tied to the meteor shower that was under way. And that can tell scientists about the shower’s parent body – a sample-return mission that never leaves Earth. The Geminids are in good view tonight. The meteors are visible from mid-evening on. At its best, the shower might produce a hundred or so meteors per hour. And you don’t need to look in a particular direction to see them – just look up and wait for the fireworks. Script by Damond Benningfield

A couple of thousand years ago, a large asteroid or comet might have been blasted apart. And we’re still seeing the fireworks from its destruction – as the Geminid meteor shower, which will reach its peak tomorrow night. Most meteor showers flare to life when Earth passes through the orbital path of a comet. The comet sheds bits of rock and dirt, which spread out along its orbit. As Earth flies through this trail of debris, the solid grains ram into the atmosphere, forming the glowing streaks known as meteors. But the Geminids are a bit odd. For one thing, their parent body – 3200 Phaethon – appears to be an asteroid or a “dead” comet, not an active comet. For another, the meteor stream contains way more material than we’d expect to see from a body the size of Phaethon. A couple of years ago, scientists came up with a possible explanation. They used observations by a Sun-orbiting spacecraft that passed through the meteor stream. They then used computer models to calculate a possible cause for the stream. They concluded that a larger body could have been destroyed. That produced Phaethon and a couple of other large remnants. But it also produced a giant cloud of dust and pebbles. So while some of the material that makes up the Geminids comes from Phaethon, a lot of it also comes from that cloud – shrapnel that makes fireworks in Earth’s night sky. More about the Geminids tomorrow. Script by Damond Benningfield

The Sun isn’t easy to influence. It’s more than a thousand times the mass of Jupiter, the solar system’s largest planet, and more than 330,000 times the mass of Earth. Even so, a recent study says the planets might influence our star’s magnetic cycle – perhaps making conditions more comfortable for life. The Sun goes through many cycles of magnetic activity. The best known lasts an average of 11 years. At the cycle’s peak, the Sun is much more active than average. It pelts Earth and the other planets with higher levels of radiation and charged particles. That can wreak havoc with everything from satellites to blood pressure. Another cycle lasts an average of less than two years. It produces “mini” peaks and valleys in the 11-year cycle. And it lines up well with the longer cycle. In the recent study, researchers from Germany compared these cycles to the orbits of the planets. They found that the peaks and valleys of the shorter cycle correspond to some planetary alignments. One was a lineup of Earth, Jupiter, and Venus. The other was an alignment of Jupiter and Saturn. The researchers said the planets may help control the solar cycles. The planets might even tamp down the Sun’s activity, which is weaker than that of many Sun-like stars. Less activity means that Earth gets bombarded by less of the nasty stuff – making our planet a much more comfortable home for life. Tomorrow: cosmic shrapnel. Script by Damond Benningfield

Storms on the Sun can cause all kinds of problems. They can knock out satellites and black out power grids. They can interfere with GPS and disrupt some radio broadcasts. They can even have an impact on human health. Solar storms happen when the Sun’s magnetic field gets tangled up. Lines of magnetic force can snap, then reconnect. That produces outbursts of radiation and charged particles. When the particles hit Earth, they’re funneled toward the surface by our planet’s own magnetic field. And that’s what causes the problems. Among the health concerns, particles and radiation can penetrate deeper into the atmosphere around the magnetic poles. That zaps anyone who’s flying at high altitudes in those regions. It’s not a fatal dose, but it’s enough to cause concerns. So airlines divert flights to avoid exposing passengers and crew. There’s also evidence that these bouts of “space weather” can boost people’s blood pressure. In one study, researchers in China looked at half a million blood pressure readings taken over six years. And they found a definite jump around the time of solar storms – especially among women and those with hypertension. An American team found similar results among older men. There’s no consensus about how space weather might cause blood pressure to spike. For now, all we know is that stormy skies on the Sun can cause lots of problems for the people on Earth. Script by Damond Benningfield

The Moon and the heart of the lion just miss each other tonight – at least as seen from the United States. As they climb into good view, after midnight, the Moon and the star Regulus will be separated by just a skosh. The farther north and east your location, the closer together they’ll appear. From some spots, they’ll be almost touching. And from much of Canada across to northern Norway they will touch – the Moon will occult the star. It’ll pass directly in front of Regulus, blocking it from view. The Moon can occult Regulus because the star lies almost atop the ecliptic – the Sun’s path across the sky. The Moon stays close to the ecliptic as well, but it does move a few degrees to either side. As a result, occultations of Regulus come in groups. This one is part of a cycle of that began earlier this year and will continue through the end of next year. Each occultation is visible from a different part of Earth. In part, that’s because each one lasts only a few minutes to a few hours, so the Moon and Regulus are below the horizon as seen from much of the world. Also, the Moon is so close to us that there’s a big difference in the viewing angle across the globe – up to two degrees – four times the width of the Moon itself. From any specific location, sometimes the angle is just right, but more often it’s a little off – providing a beautiful close encounter between the Moon and the heart of the lion. Script by Damond Benningfield

A couple of years ago, a space telescope discovered something odd about NGC 6505. The galaxy is encircled by a ring. It isn’t part of the galaxy itself. Instead, it’s an image of a background galaxy – one that’s billions of light-years farther. Einstein Rings are named for Albert Einstein because they were predicted by his theory of gravity. The gravity of a foreground object acts as a lens – it bends and magnifies the light of a background object. On small scales, gravitational lenses have revealed everything from black holes to rogue planets. Galaxies are much bigger and heavier, so they produce more dramatic lenses. Many of them create bright arcs. But when the alignment is just right, they can create a full circle. NGC 6505 is a good example. The galaxy is about twice the diameter of the Milky Way, and several times its mass. It’s about 600 million light-years away. The background galaxy is four billion light-years farther. The lensing effect has allowed astronomers to measure the amount of dark matter in the center of NGC 6505, as well as details about its stars – discoveries made possible by its beautiful ring. NGC 6505 is enwrapped in the coils of Draco, the dragon. The galaxy is more than a third of the way up the northwestern sky at nightfall. It’s visible through a small telescope. But you need a big telescope and a long exposure to make out its ring. Script by Damond Benningfield

The Moon is a “dead” world. It trembles with a few small moonquakes, and there may be occasional “burps” of gas. But for the most part, not much happens inside it. That’s definitely not the case for one of the moons of the giant planet Jupiter. Io is the most volcanically active world in the solar system. It’s covered by hundreds of volcanoes and pools of hot lava. Some of the volcanoes are larger than anything on Earth, and the lava is much hotter. The volcanoes can send gas and ash hundreds of miles high. Some of this material escapes Io completely – about one ton every second. It forms a wide “doughnut” around Jupiter. The activity is powered by a gravitational tug-of-war between Jupiter and some of its other big moons. They pull on Io in different directions. That heats Io’s interior, melting some of its rocks. A couple of recent studies found that Io has been at least this active since it was born. That suggests that Io and the other big moons have been locked into their current configuration since shortly after the birth of Jupiter itself. If that’s the case, then Io has been caught in a terrific tug-of-war for four and a half billion years. Jupiter rises above our moon this evening. The planet looks like a brilliant star – only the Moon and Venus outshine it. But you need binoculars to pick out Io and the planet’s other big moons. Tomorrow: gravitational “rings” around a galaxy. Script by Damond Benningfield

The Moon forms a beautiful grouping with the planet Jupiter and the twins of Gemini tonight. Jupiter looks like a brilliant star. It’s below the Moon as they climb into view, by about 8:30. Castor, the fainter of Gemini’s twins, is to the left of the Moon. And Pollux, the brighter twin, is to the lower left. The grouping is even tighter at first light tomorrow. The Moon circles through Gemini roughly once a month – the time it takes to complete one full turn through the background of stars. If you made a movie of those passages over the years, the Moon would look like a car that can’t stay in the same lane. That’s because the Moon’s orbit around Earth is tilted a bit compared to the ecliptic – the Sun’s path across the sky. So the Moon moves back and forth across the ecliptic during its month-long cycle. It moves from about five degrees north of it, to about five degrees south. The Moon’s position on the ecliptic relative to an individual constellation doesn’t change much from month to month. Instead, it takes years to see much of a difference. In the case of Gemini, that means that every few years the Moon comes especially close to Pollux. But then it moves away, and eventually leaves a big gap – up to the width of your fist held at arm’s length. But that won’t happen again until the early 2030s, as the Moon weaves into another lane. More about the Moon and Jupiter tomorrow. Script by Damond Benningfield

For radio astronomers, there’s some good news and some bad news. On the good side, a pilot project with SpaceX has devised a way to reduce the radio interference produced by satellites. On the bad side, the satellites can produce accidental interference. Radio telescopes tell us things about the universe that we can’t get any other way. But the telescopes are extremely sensitive. Transmissions from an orbiting satellite are like bright headlights – they overpower the subtle signals of astronomical objects. There are more than 15,000 satellites in orbit today – a five-fold increase in just six years. And the total could balloon to a hundred thousand by the next decade. Astronomers worked with SpaceX to reduce interference from its Starlink satellites. The groups combined the observing schedule of a telescope with the Starlink control system. Satellites passing over the telescope were instructed to turn away – aiming the headlights in a different direction. And there are plans to extend the scheme to other telescopes. On the other hand, a recent study found that tiny radio signals emitted by a satellite’s electronics can also be a problem. Scientists looked at 76 million radio images made by a telescope in Australia. They found that Starlink satellites interfered with up to 30 percent of the pictures. So future satellites may need extra shielding to keep them from blinding astronomy’s radio eyes. Script by Damond Benningfield

Most of the stars are so small and far away that they’re nothing more than pinpoints even in the largest telescopes. That makes it impossible to measure the size of a star. But astronomers can measure the sizes of some stars – not with a giant telescope, but with a collection of smaller ones. The technique is called interferometry. It links up several telescopes. The combo provides an especially sharp view of the heavens. If the telescopes are, say, 300 feet apart, then the combined view is as clear as that of a single telescope 300 feet in diameter. The array’s view isn’t as deep as that of a giant telescope, only as sharp. Interferometers have allowed astronomers to measure the apparent sizes of hundreds of stars. Combining that with a star’s distance provides its true size. One example is Elnath, the second-brightest star of Taurus. It’s about 134 light-years away. It’s five times the mass of the Sun. So even though it’s much younger than the Sun, it’s already passed through the prime phase of life. That’s caused it to puff up – to almost five times the Sun’s diameter. At that size, it shines more than 800 times brighter than the Sun – a big beacon for the bull. Elnath is close to the lower left of the Moon this evening. The Moon will move toward the star during the night. They’ll be closest at dawn. The gap will be smaller for skywatchers on the West Coast, and smallest for those in Alaska and Hawaii. Script by Damond Benningfield

[3, 2, 1, ignition, and liftoff of SOHO and the Atlas vehicle on an international mission of solar physics.] Generally speaking, staring at the Sun non-stop for decades is a bad idea. But a spacecraft launched 30 years ago this week has done just that. It’s told us about the Sun’s interior, its surface, and its extended outer atmosphere. That’s helped scientists develop better forecasts of space weather – interactions between Sun and Earth that can have a big effect on our technology. The craft is called SOHO – Solar and Heliospheric Observatory. It was launched into an orbit around a point in space where the gravity of Earth and the Sun are balanced. From there, its view of the Sun is never blocked. SOHO watches the Sun in many different ways. It keeps a close eye on the Sun’s magnetic field, which produces outbursts of energy and particles that can have an impact on Earth. That’s revealed shockwaves and “tornadoes” rippling across the Sun’s surface. It’s also revealed the source of the solar wind – a steady flow of charged particles that blows through the solar system. Some of SOHO’s observations block out the Sun itself, showing the space around the Sun. That’s allowed SOHO to discover more than 5,000 comets as they passed close to the Sun – many of which didn’t survive. SOHO’s mission is scheduled to end soon – closing this long-working eye on the Sun. Script by Damond Benningfield

Scientists have been searching for dark matter for decades. They haven’t found it – every experiment they’ve devised has come up empty. But they haven’t given up. Among other ideas, they’re thinking about ways to use moons, planets, and stars as detectors. Dark matter appears to make up about 85 percent of all the matter in the universe. We know it’s there because its gravity pulls on the visible stars and galaxies around it. Dark matter may consist of a type of particle that almost never interacts with normal matter. But it should interact just enough to reveal its nature. Experiments here on Earth haven’t seen any such interactions. So some scientists recommend using astronomical objects instead of lab experiments. Blobs of dark matter might enfold a binary star system. The dark matter’s gravity could pull the two stars away from each other. And dark matter might clump together to make a special kind of star. Both of those might be detectable with current telescopes. Smaller blobs might slam into an icy moon, creating a special kind of crater. Such craters could be visible on Ganymede, the largest moon of Jupiter. Two missions on their way to Jupiter might be able to see them. And dark matter might fall into the center of a planet and hang around. If enough builds up, it could heat the planet’s interior. So by studying many planets in other star systems, we might see some that are unusually warm – heated up by encounters with dark matter. Script by Damond Benningfield

Things are heating up for a planet that orbits the brightest star of Aries. The star is expanding to become a giant, so it’s pumping more energy into space. That will make temperatures extremely uncomfortable on the planet. Hamal is at the end of its life. It’s converted the hydrogen in its core to helium. Now, it’s getting ready to fuse the helium to make other elements. That’s made the core hotter. And that’s caused the star’s outer layers to puff up – to more than a dozen times the diameter of the Sun. So Hamal is about 75 times brighter than the Sun. Hamal has one known possible planet. It’s heavier than Jupiter, the giant of our own solar system. On average, the planet is about as far from Hamal as Earth is from the Sun – much closer in than Jupiter is. So every square foot of the planet’s surface receives dozens of times more energy than the same area on Jupiter does. If the planet is a ball of gas like Jupiter, then the extra heat is causing its atmosphere to puff up – and causing a lot of it to stream away into space. Over the next few million years, the planet will get even hotter, because Hamal will get even bigger. The extra energy may erode the planet’s atmosphere completely. On the other hand, the planet may spiral into the star. Either way, things are going to get much hotter for Hamal’s only known planet. Look for Hamal in the east at nightfall, well to the left of the Moon. Script by Damond Benningfield

As most parents can tell you, coming up with names isn’t easy. It sometimes takes a while to settle on something that sounds just right. It wasn’t easy for the people who named the constellations, either. Some of the names sound like they just gave up. They picked a region of the sky with few stars, gave it the name of a nearby bright constellation, then added the word “minor.” All three of these minor constellations are in good view at dawn: Ursa Minor, Canis Minor, and Leo Minor. The most famous of the bunch is Ursa Minor – the little bear. Seven of its stars form the Little Dipper, which is in the north – directly below the Big Dipper, which is part of Ursa Major. The constellation is especially well known because its brightest star is Polaris, the Pole Star. It’s at the tip of the little bear’s tail. Canis Minor is the little dog. It’s about half way up the sky in the west-southwest. It has only a couple of bright stars. The brightest is Procyon – a name that means “before the dog.” That’s because the little dog leads the big dog across the sky. In ancient Greece, in fact, the constellation was known as Procyon. Finally, Leo Minor is high overhead. It’s the little lion, standing on the shoulder of Leo. That region of the sky wasn’t depicted as a separate constellation until 1687. Today, though, it’s one of the 88 official constellations – even if it is a “minor” one. Script by Damond Benningfield

The shortest season on the planet Mars begins today – autumn in the northern hemisphere, and spring in the southern hemisphere. It will last for 142 Mars days – almost eight weeks less than the longest season. Mars has seasons for the same reason that Earth does – it’s tilted on its axis. And the tilt is at almost the same angle as Earth’s. But the seasons on Mars are more exaggerated because the planet’s orbit is more lopsided. A planet moves fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. That stretches out some seasons, and compresses others. It also changes the intensity of the seasons. Mars is farthest from the Sun when it’s summer in the northern hemisphere. So northern summers are fairly mild, while southern winters are bitterly cold. On the flip side of that, northern winters are less severe, while southern summers are the warmest time on the whole planet. The start of northern autumn also marks the beginning of dust-storm season. Rising currents of air can carry along grains of dust. Enough dust can be carried aloft to form storms that cover thousands of square miles. And every few Martian years, a storm gets big enough to cover the entire planet. The storms usually peak around the start of southern summer. Mars is about to pass behind the Sun, so it’s hidden in the Sun’s glare. It’ll return to view, in the dawn sky, in early spring – on Earth. Script by Damond Benningfield

The Moon slides by Saturn the next couple of nights. The planet looks like a bright star. It’s to the left of the Moon as night falls this evening, and to the lower right of the Moon tomorrow night. Saturn is best known for its rings. They’re almost wide enough to span the distance from Earth to the Moon. Right now, we’re viewing them almost edge-on, so they look like a thin line across the planet’s disk. Saturn isn’t the only world with rings. The solar system’s three other giant outer planets also have them. But they’re dark and thin, so they’re hard to see. Several asteroids and dwarf planets have rings, too. But the biggest set of rings yet seen may encircle a “rogue” planet about 450 light-years away. The possible rings were discovered years ago. Over a period of eight weeks, the light of a star in Centaurus flickered – sometimes dropping to just five percent of its normal level. The most likely cause was the passage of a set of rings in front of the star. And it’s quite a set. The rings are more than a hundred million miles across – greater than the distance from Earth to the Sun. The ringed planet appears to be traveling through the galaxy alone, and it just happened to pass in front of the star. It could be up to six times the mass of Jupiter, the giant of our own solar system. And moons could be orbiting inside the rings – the most impressive rings we’ve seen anywhere in the galaxy. Script by Damond Benningfield

Planets are tough little buggers. They can form and survive in some extreme environments. In fact, the first confirmed planets outside our own solar system orbit the remnant of a dead star – a pulsar. A pulsar is tiny – the size of a small city. But it’s more massive than the Sun. A teaspoon of its matter would weigh as much as a mountain. Yet a pulsar spins rapidly – up to several hundred times per second. It has an extreme magnetic field. The field shoots “jets” of particles out into space. As the pulsar spins, the jets can sweep across Earth like a lighthouse beacon, producing short pulses of energy. The timing of those pulses is extremely precise. That makes pulsars some of the best clocks in the universe. But the timing can be changed by a companion – another star, or even a planet. And that’s how pulsar planets are discovered – through tiny changes in the timing of the pulses. Eight pulsar planets have been confirmed. But they present quite a challenge. A pulsar is the remnant of a titanic explosion – a supernova. It’s hard to see how any planets could survive such a blast. So it’s likely that the planets formed after the blast – perhaps from debris from the explosion’s aftermath. Regardless of how they formed, the planets aren’t friendly places. They’re blasted with charged particles, X-rays, and gamma rays from the pulsar. That may slowly erode the planets – no matter how tough they are. Script by Damond Benningfield

[pulsar audio] This is the rhythm of the stars – the beat of dead stars. It’s the “pulses” of radio waves produced by rapidly spinning stellar corpses. They produce beams of energy that sweep around like the beacon of a lighthouse. Radio telescopes detect the beams when they sweep across Earth. The stars are known as pulsars. They’re some of the most extreme objects in the universe. They’re neutron stars – the dead cores of some of the most massive stars. When a heavy star can no longer produce nuclear reactions in its core, the core collapses. Gravity squeezes the core down to the size of a small city. But that tiny ball is heavier than the Sun. The star is rotating as it dies. As the core collapses, it keeps on spinning. But the smaller it gets, the faster it spins. So newborn neutron stars can spin a few dozen to a few hundred times per second. Particles trapped in the neutron star’s magnetic field produce energy that’s beamed into space – the source of the pulses. The neutron star spins down over time, slowing the pulses. But if it has a close companion, it can be revved up even faster. The neutron star can pull gas from the surface of the companion. As it hits the neutron star, the gas acts like an accelerator – creating some of the fastest pulsars in the universe. These extreme stars can still host planets; more about that tomorrow. Script by Damond Benningfield

Getting too close to a black hole is bad news. The black hole’s gravity can pull apart anything that’s falling into it atom by atom. A magnetar can do the same thing. And it’s not just its gravity you have to worry about. Its magnetic field can do the job as well – from hundreds of miles away. A magnetar is a neutron star -the crushed corpse of a once mighty star. It’s heavier than the Sun, but only a little bigger than Washington, D.C. It’s born when a massive star can no longer produce nuclear reactions in its core. The core collapses, while the star’s outer layers explode. The original star generated a strong magnetic field. As the core collapsed, the field was mashed inward as well, making it extremely powerful. It’s boosted by the turbulent sloshing inside the newly formed neutron star. So a typical magnetar’s magnetic field is a million billion times the strength of Earth’s field. The neutron star sticks around, but its magnetic field weakens in a hurry. So there aren’t many magnetars around – only about 30 have been discovered. The magnetic field can help produce titanic explosions. Interactions with the field can cause the crust of a neutron star to crack in a “starquake.” Energy from the quake is beamed out by the magnetic field, producing an outburst of gamma rays. The most powerful quake yet seen generated more energy in a tenth of a second than the Sun will emit in 150,000 years – the enormous power of a magnetar. More about neutron stars tomorrow. Script by Damond Benningfield

When the most massive stars die, they can leave behind two types of corpse. The heaviest ones probably form black holes. But the fate of the others is no less exotic. They form neutron stars – ultra-dense balls that are more massive than the Sun, but no bigger than a small city. A massive star “dies” when its core can no longer produce nuclear reactions. For a star of about eight to 20 or more times the mass of the Sun, the core collapses, while the star’s outer layers explode as a supernova. The gravity of the collapsing core squishes together protons and electrons to make neutrons – particles with no electric charge. The neutrons can be squished together only so much before they halt the collapse. By then, the core is trillions of times as dense as Earth. So a chunk of a neutron star the size of a sugar cube would weigh as much as a mountain. A neutron star probably has a solid crust made of iron or other elements, with no features more than a couple of millimeters tall. The gravity at the center of a neutron star is so strong that we don’t really know what the conditions are like – there’s just nothing to compare it to. There could be as many as a billion neutron stars in the galaxy. But they’re hard to find. Some of them make it a little easier, though. They produce the most powerful magnetic fields in the universe – and some of the most powerful outbursts. More about that tomorrow. Script by Damond Benningfield

You can always count on the constellations. Over the course of a human lifetime, their configuration doesn’t change – they don’t appear to move at all. That’s an illusion, though. The stars are all so far away that we don’t see any motion. But they’re all moving in a hurry. And one of the fastest is in view on autumn evenings. Gamma Piscium is the second-brightest member of Pisces, the fishes. The constellation stretches across the east and southeast at nightfall. Gamma Piscium is near its top right corner – part of a pentagon of faint stars. Gamma Piscium is a giant. It’s nearing the end of its life, so it’s getting bigger and brighter. Right now, it’s about 10 times the diameter of the Sun, and more than 60 times the Sun’s brightness. That makes it faintly visible to the eye alone, even though it’s 135 light-years away. Perhaps the most interesting fact about Gamma Piscium is its speed: It’s moving through the galaxy at about 340,000 miles per hour – faster than all but a few other visible stars. At that rate, it’ll move the equivalent of the Moon’s diameter in less than 3,000 years. The star’s composition hints that it came from outside the disk of the Milky Way – the part of the galaxy that includes the Sun. The star has very few heavy elements. That suggests it formed outside the disk, and just happens to be passing by – zipping through the galaxy like a speeding rocket. Script by Damond Benningfield

An interloper from another galaxy scoots low across the south on October evenings. It’s a tight family of stars – hundreds of thousands of them. The stars probably belonged to another galaxy that was consumed by the Milky Way in the distant past. Messier 30 is low in the south at nightfall, in Capricornus. The sea-goat’s brightest stars form a wide triangle. M30 is on the lower left side of the triangle Messier 30 is a globular cluster – a ball of stars about 90 light-years wide. Most of the stars are concentrated in the cluster’s dense core. The numbers tail off as you move toward the cluster’s edge. Anything that wanders too far from the center gets yanked away by the gravity of the rest of the galaxy. The Milky Way is home to more than 150 globular clusters. But several of them appear to have come from other galaxies. And that includes M30. The main clue to its origin is its orbit. As it circles the center of the galaxy, M30 moves in the opposite direction from most of the stars and star clusters. The only way for such a massive cluster to move against the traffic is if it came from outside the galaxy. So Messier 30 isn’t a native of the Milky Way. Instead, it was pulled in by the Milky Way’s powerful gravity – making it a refugee from another galaxy. We’ll talk about an individual star that might be a refugee from another part of the galaxy tomorrow. Script by Damond Benningfield

If you’ve ever left a can of soda in the freezer for too long, you can appreciate what happened to the largest moon of the planet Uranus: It cracked. Titania is almost a thousand miles in diameter – less than half the size of our moon. But it orbits Uranus at about the same distance as the Moon does from Earth. And like the Moon, it’s locked in such a way that the same hemisphere always faces its planet. When Titania was born, its interior was warm. But it quickly froze. As it did so, the surface cracked, creating some impressive canyons. The largest is a network known as Messina Chasma. Like Titania itself, it’s named for a character from Shakespeare – in this case, from “A Midsummer Night’s Dream`.” The canyons are more than 900 miles long, wrapping from the equator to near the south pole. They’re up to 60 miles wide, and miles deep. Few impact craters have scarred Messina, indicating that it’s fairly young. In fact, Titania’s entire surface appears to be younger than those of Uranus’s other big moons. That doesn’t mean the moon itself is younger. Instead, it probably was repaved by ice flowing from inside – resetting the clock for this fractured moon. Uranus is in view all night, in Taurus. And it’s closest to Earth for the year – 1.7 billion miles away. Despite the distance, it’s big enough that it’s an easy target for binoculars. But you need a decent telescope to see Titania. Script by Damond Benningfield